The present invention is directed to a solid state control device for controlling the turn on and turn off of electrical loads such as incandescent lights. In particular, the invention provides for gradual turn on and turn off of a light bulb to increase the bulb life.
The present invention relates to solid state control of light bulbs and the elimination of problems associated with normal mechanical contact on/off switching devices. It is known in the prior art to use solid state switching devices such as silicon controlled rectifiers (SCR) or triacs for light control and dimming. Since these solid state switching devices readily lend themselves to control over most of the AC sine wave, these devices are ideal for this type of application. However, little thought has been given to the concepts of possible energy savings via the wall switch or to increasing the life of the light bulbs.
Naturally, an energy savings can be effected by dimming an electrical load such as a light bulb. Likewise, bulb life can be extended by operating the light bulb at a reduced voltage either by use of a dimmer device or by use of any one of a number of devices which block a portion of the AC wave form, such as a diode in series with the light bulb. However, the use of such devices for energy savings or to extend bulb life is suspect. For example, typical energy savings devices reduce light output enough so that a higher wattage bulb must be used thereby negating the energy savings.
In nearly 70% of all light bulb applications, it would be possible to control the period of time the light bulb is used by automatically switching it off after a predetermined period of time. Control in this fashion would be particularly useful if it were possible to override the timed turn off feature. Typical applications for a switch of this type would include lights for closets, halls, garages, stairwells, porch and bathroom. Nearly any light bulb could be controlled in this manner if the timed turn off period is adjustable.
An additional feature of a solid state control device of this type would be to extend bulb life by means of controlled turn on as well as controlled turn off. The electrical resistance of the bulb filament is quite low when the filament is cold, normally on the order of 10 to 30 ohms (dependent upon bulb wattage). As the filament heats, this resistance increases to an order of 150 to 200 ohms which is a factor greater than ten to one. This low initial resistance causes a tremendous surge of current at turn on which coincidentally is when most bulbs burn out. The repeated heating and cooling of the filament, coupled with current surges put through it, causes bending and weakening of the filament which eventually results in failure of the light bulb.
Several circuits are known in the prior art to effect the gradual turn on and turn off of electrical loads such as incandescent light bulbs. For example, the United States Pat. No. 4,008,416 issued on Feb. 15, 1977 to Henry H. Hakasone, several circuits are shown for providing gradual turn on and turn off of an incandescent light bulb. Some of these circuits employ a thermistor which changes the conduction angle of a solid state device to gradually apply power to the bulb to extend bulb life. This patent also shows two circuits in FIGS. 5 and 6 which use capacitors to effect a gradual turn on and turn off of the bulb. In particular, in FIG. 5, a capacitor C5 gradually charges to increase the conduction angle of diacs 109 and 110 to eventually trigger the triac 111 which results in a gradual increase in the intensity of the light bulb L1. Gradual turn off of the light bulb L1 is accomplished by gradually decreasing the charge on the capacitor C5 by discharging the capacitor through resistors 112 and 113. Similarly, with respect to FIG. 6, capacitors C8 and C9 are charged and discharged to gradually turn on and turn off the light bulb L3. Although these latter circuits accomplish both gradual turn on and turn off of the light bulb, a large number of components is required including multiple triacs, resistors, capacitors, etc.
In a subsequent U.S. Pat. No. 4,152,608 issued to Nakasone, et al, on May 1, 1979, several improvements were made in the above described circuits. This later Nakasone, et al, patent describes a circuit which gradually changes the amount of AC power applied to a light bulb following a change in the state a bi-stable switch. The circuit includes a charging capacitor which is charged and discharged upon actuation of the bi-stable switch to turn a triac on and off to actuate the light bulb. A transistor circuit is connected to the capacitor to charge and discharge the capacitor in accordance with the position of the bi-stable switch.
In addition to the above circuits, several other patents disclose circuits for controlling the energization of an electrical load. In U.S. Pat. No. 4,082,961 issued on Apr. 4, 1978 to Luther L. Genuit, several turn off circuits use an SCR or triac which is responsive to a decrease in the charge on a capacitor to gradually change its conduction angle. As the SCR or triac fires at later points in the half cycle of the applied AC signal, the light bulb is gradually turned off. The purpose of this circuit is to energize the light bulb for a period of time after turning off the light switch. In U.S. Pat. No. 3,991,343 issued on Nov. 9, 1976, to Henry Delpy, a light control circuit is shown for progressively varying the illumination of a lamp by charging or discharging several capacitors connected to an SCR. However, the on/off cycle of the light control circuit is automatic and is not directly responsive to the position of the switch.
While the above patents show electronic switching arrangements for controlling the turn on and turn off of electrical loads, other types of circuits are known in the art. For example, traditional light dimmer switches employ potentiometers to control the power applied to an electrical load such as a light bulb. Many different modifications of these traditional dimmer type control circuits are known in the art using various combinations of RC circuits, transistors and SCRs.
As evident from the above discussion, all the known prior art circuits use a large number of electrical components to achieve relatively simple control features. Since a premium is placed on the number of components in any such circuit because of the ultimate consumer costs, it is highly desirable to minimize the number of components while maintaining a high degree of reliability. Although the prior art recognizes the need for simplicity, the known circuits use an excessive number of components.